1. Technical Field
This invention relates to a power supply device. The present invention relates to a power supply device used for a back light of a liquid crystal display device, and an electronic device such as a personal computer. Particularly, the present invention relates to an uninterruptible power supply (UPS) which supplies power when an input voltage from a power supply accidentally drops below a predetermined value.
2. Related Art
A power supply circuit used for an electronic device is required to maintain feeding electric power for a predetermined time period after an electric failure such as an abrupt voltage drop and/or an instantaneous interruption occurs. If the electronic device is fed at least for the predetermined time period after the above electric failure, malfunction and/or breakdown of the electronic device caused due to the above electric failure can be prevented. Hence, installation of an auxiliary power supply is indispensable for protecting the electric device from the electric failure.
For example, Japanese Patent Application Publication HEI 08-298769 (JP 08-298769) discloses a switching power supply having an auxiliary power supply. The power supply device is proposed in which a secondary winding is added to a reactor constituting an active filter in a step-up chopper, and a higher frequency AC voltage is taken out from the secondary winding as an input of the auxiliary power supply. The switching power supply includes the auxiliary power supply so that activation and suspension of the power supply can be controllable from an external device.
However, the power supply device does not have any mechanism for ensuring the sufficient holding time to maintain the feed of the electric power by the power supply device. As a result, a problem may arise that the feed from the power supply device is stopped.
On the other hand, an insulation type of power supply device is disclosed for insulating an input side from an output side through a transformer provided therebetween. This power supply device includes a capacitor on a secondary side of the transformer for storing electric energy and ensuring feed of electric power to a load for a certain time period after an instantaneous interruption, i.e., a holding time period.
However, the insulation type of power supply device has the following possible disadvantages. Generally, the secondary side of the transformer in the power supply device has a lower voltage than a voltage of the primary side. Energy stored in the capacitor is expressed by CV2/2 (C is a capacitance of the capacitor, and V is a voltage across the capacitor), and is proportional to the square of the voltage across the capacitor. Since the voltage of the secondary side is generally low, a larger capacitance of the capacitor is required to ensure the desired holding time period. This structure results in upsizing of the power supply device.
An object of the present invention is to provide a power supply device which ensure sufficient feed of electric power to an electric and/or electronics device connected thereto for a certain time period after an input voltage to the power supply device falls to or less than a predetermined value.